As an example of a continuous variable speed transmission used in a manner to be mounted on an agricultural vehicle or the like, there is well known a belt-type transmission comprising a belt variable speed mechanism. In the belt variable speed mechanism, an input shaft and an output shaft, placed in parallel with each other, are each provided with a variable speed pulley composed of (a) a fixed sheave mounted on the shaft for unitary rotation and for no movement along the shaft and (b) a movable sheave which is opposed to the fixed sheave so as to form a V-shaped belt groove therebetween and is supported to the shaft for unitary rotation and for movement along the shaft, and a variable speed belt is wound between the belt grooves formed in both the variable speed pulleys on the input and output shafts. The movable sheaves of the variable speed pulleys are moved along the input and output shafts, respectively, so that respective effective radiuses of the variable speed pulleys with respect to the variable speed belt are changed. As a result, a speed ratio between the input and output shafts is changed.
Japanese Patent Application Laid-Open Gazette No. 62-118159 proposes a continuously variable speed transmission in which a belt variable speed mechanism having the above structure and a gear mechanism are arranged in parallel between input and output shafts and a planetary gear mechanism as a differential gear mechanism is disposed among the belt variable speed mechanism, the gear mechanism and the output shaft.
In the proposed continuously variable speed transmission, the planetary gear mechanism has three gear elements, the first gear element is connected to the gear mechanism thereby constituting a power transmission path, the second gear element is connected to the belt variable speed mechanism thereby constituting another power transmission path, and the third gear element is connected to the output shaft. More specifically, first and second rotation shafts are placed coaxially with the output shaft, the gear mechanism is provided between the first rotation shaft and the input shaft, and the first gear element is mounted on the first rotation shaft. Further, the belt variable speed mechanism is provided between the second rotation shaft and the input shaft, and the second gear element is mounted on the second rotation shaft. By changing the rotational speed of the gear element connected to the belt variable speed mechanism while rotating the gear element connected to the gear mechanism at a constant rate, the rotational speed of the output shaft is changed or the rotational direction of the output shaft is changed.
In the continuously variable speed transmission having the differential gear mechanism such as the above-mentioned planetary gear mechanism, when the output shaft is rotated from rest, the power transmission path is divided into a driving power transmission path and an inverse power transmission path between the gear mechanism and the belt variable speed mechanism. In other words, when one path becomes a driving power transmission path, the other becomes an inverse power transmission path. Which becomes the driving power transmission path or the inverse power transmission path depends upon the difference in circumferential speeds between the gear element connected to the gear mechanism and the gear element connected to the belt variable speed mechanism. The path including the gear element having a larger circumferential speed becomes the driving power transmission path. Since driving power and inverse power arise as power, power obtained by subtracting the inverse power from the driving power is output power.
However, in the continuously variable speed transmission in which the belt variable speed mechanism and the gear mechanism are parallel-arranged between the input and output shafts as in the above-proposed example, when the distance between both the shafts is changed, this requires a design change such that the size of each gear of the gear mechanism and the number of gears thereof are changed, which increases the cost.
Further, in order to lubricate, with a lubricant, parts among elements of the differential gear mechanism and mating points of the gear mechanism, casings for keeping the lubricant are necessary. Finding spaces for disposing the casings prevents the continuously variable speed transmission from being compacted.
To cope with these problems, it can be considered that a belt power transmission mechanism is used instead of the gear mechanism. More specifically, the input shaft and the first rotation shaft are each provided with a pulley for unitary rotation, and a power transmission belt is wound between both the pulleys. According to this arrangement, since both the two power transmission paths each include the belt, a change in the center distance requires only a belt change. Further, since there is no need for lubrication as in the case of the gear mechanism, this eliminates the need for casings for lubrication.
Meanwhile, in the case of using the belt power transmission mechanism, maintainability is important unlike the case of using the gear mechanism. That is to say, it is desirable to make maintenance work such a belt inspection and a belt change readily feasible. For example, the presence of a casing containing the differential gear mechanism may impair maintainability.
Generally, in the drive train of a vehicle, a reduction gear mechanism is incorporated immediately forward of a drive shaft connected to a drive wheel. When the above continuously variable speed transmission is placed at a part on the drive shaft side, sufficient consideration must be given to the positional relationship between the continuously variable speed transmission and the reduction gear mechanism.
Accordingly, in a continuously variable speed transmission of the present invention in which one of two power transmission paths connecting to a differential gear mechanism such as a planetary gear mechanism is formed of a belt variable speed mechanism and the other power transmission path is formed of a belt power transmission mechanism instead of a gear mechanism, an object of the present invention is to improve the positional relationship between the belt power transmission mechanism and the casing of the differential gear mechanism or the reduction gear mechanism of the vehicle and the positional relationship between the belt variable speed mechanism and the casing of the differential gear mechanism or the reduction gear mechanism of the vehicle thereby obtaining good maintainabilities of both the belt power transmission mechanism and the belt variable speed mechanism and further accelerating the application of the continuously variable speed transmission to vehicles.